Wireless LAN has drawn attention as a system that liberates a user from LAN wiring of a wired system. According to the wireless LAN, since most wired cables can be eliminated in a work area such as an office, a communication terminal such as a personal computer (PC) may be moved without much difficulty. In recent years, the faster and less expensive the wireless LAN system becomes, the further the demand increases. Particularly, in recent, an introduction of a personal area network (PAN) is proposed for constructing a small-scaled wireless network among a plurality of electronic devices surrounding a person so as to perform information communications among these devices. For example, provisions are set for different wireless communication systems and wireless communication devices, which utilize frequency bands that do not require a license from a regulatory agency, such as a 2.4 GHz band and a 5 GHz band.
For example, in recent years, a system in which wireless communications are performed with information carried in a very weak impulse stream, called “Ultrawide Band (UWB) communication”, has drawn attention as a wireless communication system that realizes a short-range ultra-fast transmission, and its practical application has been expected. At present, in IEEE 802.15.3 and the like, a data transmission system with a packet structure including a preamble as an access control system for ultrawide band communication is devised.
In the case where the wireless network is constructed in a work environment where many devices coexist indoors, it is assumed that a plurality of networks is constructed in an overlapped manner. In the wireless network using a single channel, even if another system interrupts during communication or even if communication quality is reduced due to interference or the like, there is no room for mending the situation. Therefore, a multichannel communication system in which a plurality of frequency channels are prepared to perform frequency hopping, thereby operating is considered. For example, when the communication quality is reduced due to interference or the like during communication, the network operation is maintained by frequency hopping, so that coexistence with other networks can be realized.
Furthermore, when the wireless network is constructed indoors a multipath environment in which a receiving apparatus receives the combination of a direct wave and a plurality of reflected waves/delayed waves is formed. Delay distortion (or frequency selective fading) is generated by the multipath, thereby causing an error in communication. Further, there occurs inter symbol interference attributed to the delay distortion.
As a main countermeasure against the delay distortion, a multicarrier transmission system can be exemplified. In the multicarrier transmission system, since transmission data is divided into a plurality of carriers with different frequencies, the band of each of the carriers becomes a narrow band, which makes it difficult to be affected by the frequency selective fading.
For example, in an OFDM (Orthogonal Frequency Division Multiplexing) system, which is one of the multicarrier transmission systems, frequencies of respective carriers are set so that the respective carriers are mutually orthogonal within a symbol block. At the time of information transmission, information sent serially is subjected to serial/parallel conversion every symbol period which is slower than an information transmission rate and a plurality of pieces of outputted data are assigned to the respective carriers to modulate the amplitude and the phase for each of the carriers, and inverse FFT is applied to the plurality of carriers, so that the carriers are converted into time-base signals while holding the orthogonality of the respective carriers on a frequency basis to be sent. Furthermore, at receiving time, a reverse operation is performed, that is, FFT is performed to convert the time-base signals into the frequency-base signals, demodulation of the respective carriers according to the respective modulation systems thereof is performed, and parallel/serial conversion is performed to reproduce the original information sent in a serial signal.
The OFDM modulation system is employed as a standard of wireless LAN, for example, in IEEE 802.11a/g. Also, in IEEE 802.15.3, in addition to a DS-UWB system in which diffusion speed of an information signal of DS is increased to the maximum, and an impulse-UWB system in which an information signal is composed using an impulse signal stream with a very short period of about several 100 picoseconds to be transmitted and received, standardization of an UWB communication system employing the OFDM modulation system has been developed. In the case of the OFDM_UWB communication system, there has been considered OFDM modulation using IFFT/FFT, in which frequency hopping (FH) every a plurality of subbands each having a 528 MHz width is applied to frequency bands of 3.1 to 4.8 GHz, and the frequency bands are composed of 128 points (for example, refer to Non-Patent Document 1).
In FIG. 7, frequency assignment defined in the multiband OFDM_UWB communication system is shown. As shown the same figure, the assignment is such that a group A is composed of bands #1 to 3 having center frequencies of 3432 MHz, 3960 MHz, and 4488 MHz, respectively, a group B is composed of a band #4 and a band # 5 having center frequencies of 5016 MHz and 5808 MHz, respectively, a group c is composed of bands #& to # 9 having center frequencies of 6336 MHz, 6864 MHz, 7392 MHz and 7920 MH, respectively, and a group D is composed of groups #10 to #13 having center frequencies of 8448 MHz, 8976 MHz, 9504 MHz and 10032 MHz.
In the multiband OFDM_UWB system, the center frequencies corresponding to these respective bands need to be synthesized. Among them, the use of three bands of the group A is mandatory and the use of seven bands of the group A and the group C is defined as an option. The other groups and the bands are prepared for extension in future.
Although for frequency switching, it is generally considered to multiply the same oscillation frequency by a PLL (phase Lock Loop), the multiband OFDM_UWB system has a problem that the switching width of the channel as shown in FIG. 7 is large and the frequency switching in such a wide band cannot be performed by a single PLL.
Furthermore, by providing a plurality of oscillators so that each of them generates a frequency band, a high precision multiband generator can be constructed. However, there is a problem with circuit area and power consumption. Accordingly, there is technical demand for making a plurality of frequency bands from a single oscillator by frequency division.
For example, by repeating the frequency division of a single frequency outputted from an oscillator, mixing is applied to the respective frequency division outputs (that is, either of a sum and a difference of the frequencies is outputted) to thereby perform multiband generation.
In FIG. 8, a conventional example of a frequency synthesizing block for hopping (which is 3-band mode of the group A) used in the multiband OFDM system is illustrated (for example, refer to Non-Patent Document 1). The center frequency of each band can be synthesized (frequency addition/subtraction), using frequency division of a reference frequency obtained from a single oscillator (for example, TCXO) and mixers, as shown in the figure.
In the example shown in the same figure, a frequency 4224 MHz obtained by multiplying an oscillation frequency outputted by the oscillator by a PLL is a reference frequency. First, a frequency of 528 MHz is taken out by ⅛ frequency division and a frequency of 264 MHz is taken out by ½ frequency division. Furthermore, the frequency of 528 MHz necessary as a sample clock can be synthesized by frequency division.
Subsequently, in each mixer indicated by SSB (Single Side Band), frequency addition is performed using 528 MHz and 264 MHz, resulting in a frequency of 794 MHz. In addition, one of 264 MHz and 794 MHz is selected by a selector (Select), and a frequency of 3960 MHz can be obtained as a desired center frequency by frequency subtraction of 4224 MHz and 264 MHz in the SSB at a latter state and a frequency of 4488 MHz can be obtained by frequency addition of 4224 MHz and 264 MHz. Further, a frequency of 3422 MHz can be obtained by subtracting 792 MHz from 4224 MHz.
In each of the mixers indicated by SSB in FIG. 8, orthogonal components with respect to each other are prepared in each signal and frequency synthesis can be performed by addition and subtraction of the frequencies using the addition theorem of the trigonometric function as shown in the below-described equation. In FIG. 9, a configuration of a frequency adder is shown. Here, a function rot(x) is defined as rot(x)=exp(2πjx).
Furthermore, in FIG. 10, a conventional example of a frequency synthesizing block used in 7-band mode composed of the group A and the group C is illustrated. Center frequencies of respective bands, as shown in the figure, can be synthesized (frequency addition/subtraction) using frequency division of a reference frequency obtained from a single oscillator (for example, TCXO) and mixers.
In the example of the same figure, a frequency 6336 MHz obtained by multiplying an oscillation frequency outputted from the oscillator by a PLL is a reference frequency. First, a frequency of 2112 MHz is taken out by ⅓ frequency division and then, a frequency of 1056 MHz is taken out by ½ frequency division. Furthermore, frequencies of 528 MHz and 264 MHz are taken out by repeating ½ frequency division twice. In addition, the frequency of 528 MHz necessary as a sample clock can be synthesized by frequency division.
Moreover, in the example of the same figure, five SSB blocks that performs frequency addition/subtraction are mounted. In a first SSB, a frequency of 4224 MHz is obtained by frequency subtraction of 6336 MHz and 2112 MHz. In a second SSB, a frequency of 1584 MHz is obtained by frequency addition of 1056 MHz and 528 MHz. Further, in a third SSB, frequency addition/subtraction of any one of 1056 MHz, 1584 MHz and 528 MHz, which is selected by a selector, and 6336 MHz is performed. In a fourth SSB, a frequency of 792 MHz is obtained by frequency addition of 528 MHz and 264 MHz. Furthermore, in a fifth SSB, the 4224 MHz obtained by the first SSB is subjected to frequency addition/subtraction of either 792 MHz or 264 MHz, which is selected by the selector. Finally, the frequency addition/subtraction result of the third SSB or the fourth SSB is selectively outputted, and consequently, the center frequencies of seven bends composed of the group A and the group C can be obtained.
(Non-Patent Document 1)
IEEE 802.15.3a TI Document <URL:http://grouper.ieee.org/groups/802/15/pub/2003/May03, file name:03142r2P802-15 TI-CFP-Document.doc>